Semi-solid forming of metal-matrix nanocomposites

Abstract

A metal matrix nanocomposite is formed by heating a metal above its liquidus temperature, adding nanoparticles, and then agitating the mixture with high-frequency (and preferably ultrasonic) vibration. The mixture can then be cooled below the liquidus of the metal to a semisolid state, and placed in a mold to form it into some desired shape. The formed mixture can then be quenched or otherwise allowed to cool to provide an article in finished (or nearly so) form.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH[0001]This invention was made with United States government support awarded by the following agencies:[0002]National Science Foundation (NSF) Grant No. 0323509The United States government has certain rights in this invention.FIELD OF THE INVENTION[0003]This document concerns an invention relating generally to metal forming processes, and more specifically to methods for performing semisolid casting.BACKGROUND OF THE INVENTION[0004]High-pressure die casting (HPDC) is a process where metal (this term being used to refer to both metals and alloys) above its liquidus temperature, i.e., fully liquid metal, is injected into a cavity in a mold at high speed and pressure. HPDC is one of the most economical processes for mass production of cast metal items because of the short time needed for the casting cycle, the ability to make multiple items in a single casting, and the fact that the items leaving the mold may be in final form (or nearly so...

AI Technical Summary

Benefits of technology

[0011]A metal matrix nanocomposite, that is, a composite wherein small particles (having at least one dimension on the order of 100 nm or less) are dispersed throughout a metal matrix, can be formed by heating a metal above its liquidus temperature, thereby placing the metal in its liquid state. Nanoparticles may then be added to the liquid metal, preferably in amounts of 0.25%-5.0% of the weight of the overall mixture, and most preferably in amounts of 0.25%-2.0% of the weight of the mixture. The metal / nanoparticle mixture can then be agitated by subjecting the mixture to high-frequency (preferably >5 khz) vibration. Most preferably, ultrasonic vibration (>20 khz) is used, and is applied by at least partially inserting a vibrating probe / member into the mixture (or possibly multiple probes, e.g., where the volume of the mixture is large). Such vibration has been found effective to at least substantially uniformly disperse the nanoparticles throughout the mixture, usefully reducing or eliminating problems with clumping of the nanoparticles.

[0012]The mixture can then be cooled, and the vibrating member can be removed from the mixture. Preferably, at the latest, the vibrating member is removed when the metal is at or near its liquidus temperature (e.g., within about 50 C of the liquidus). Removal of the vibrating member can also occur after the liquidus is reached, but this is preferably avoided since it can promote solidification and build-up of the mixture on the vibrating member. It has been found that when the mixture is then cooled below the liquidus temperature of the metal (but above the metal's solidus temperature), it has an at least substantially globular microstructure which beneficially allows the use of semisolid casting: the mixture can be situated in a mold and formed into a desired shape while it is partially solidified (preferably while it has a solid fraction between 40%-70%), and it maintains a stable, nonturbulent flow front even when urged into the mold cavity at high velocity. Usefully, an at least substantially globular microstructure is maintained even where the mixture is quenched after solidification begins (i.e., where the mixture is rapidly cooled by immersion of the formed item and / or its die in a liquid bath, usually of water, oil, or molten salts). This result is surprising since ordinarily the temperature gradients arising from quenching promote a dendritic (non-globular) microstructure, thereby causing casting difficulties and / or nonuniformity in the strength and other properties of the casting.

[0013]The use of vibrational agitation is believed to assist with dispersion of nanoparticles throughout the (molten) metal matrix, and also with generation of a globular microstructure by fragmenting any forming dendrites. The nanoparticles, as well as any dendrite fragments, then serve as dispersed nucleation points during solidification, thereby helping to create and maintain the globular microstructure. Machine parts or other articles formed by use of the process have improved properties owing to the presence of the nanoparticles, and at the same time the advantages and efficiencies of semisolid casting are realized. Additionally, the ability to quench the cast article to room temperature once solidification begins saves significant time and energy over competing semisolid casting processes, wherein slower or temperature-controlled cooling may be needed.

Problems solved by technology

This result is surprising since ordinarily the temperature gradients arising from quenching promote a dendritic (non-globular) microstructure, thereby causing casting difficulties and / or nonuniformity in the strength and other properties of the casting.

Additionally, the ability to quench the cast article to room temperature once solidification begins saves significant time and energy over competing semisolid casting processes, wherein slower or temperature-controlled cooling may be needed.

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